Master Encoder to Virtual

Hi,

I have a motion control issue that I been trying to solve.

Firstly the machine master axis run by a 3-phase motor via inverter and attached to a physical rotary encoder. However the full rotation of the encoder are of varying load, so we not able to achieve constant velocity over the full rotation.
I know that to get constant velocity we just need to replace the inverter with a servo drive, but this is not an option (Cost and Boss)
when the master is run at relatively high speed the problem is not evident, only in slow speed that it become a problem. It is not something that need to be solve but better to solve it for the machine.

So I am think of software solution, by creating a virtual axis that have the same speed as the real encoder, and use additive positioning offset to the virtual when both the real and virtual axis is out of phase.

For those of you who have similar problem, will this method work?

Regards

you need decide what method to use whether close loop or open loop.

if using to simulate as encoder the potential of danger will arise.

I suppose an close loop, I was thinking that when the master axis reaches it home position (0 degrees) then it will check the position of the virtual axis, if it
not 0 +-1 degree then an additive position offset to be apply to the virtual to make it 0 either in 1 step or several step.

Regards

Anything you over ride the basic principle will invite a problem.

please check/anaylis your process

the system will follow the process to meet the task.

what are you using the master axis for?
ie what are you controlling and what is the effect on the machine at slow speeds that you do not like?

Have you checked the inverter tuning? what control method is the inverter using (closed loop vector or open loop)
What is the inverter brand and model?

the master axis is use to drive a cam, which in turn drive a vertical shaft up and down. Like what shown in the image.

So you can see that when the cam is reaching the peak point, the load on the axis is the greatest hence the master axis drive slow down and when the peak is over the speed will pick up again.

Then I cannot get consistent speed over 1 revolution of the cam. Especially at slow speed on the master axis.

The inverter we use for the master axis is an AB powerflex 40.
I have try to use the parameter A084 [Boost Select] but not success.

Regards

sohwel said:

the master axis is use to drive a cam, which in turn drive a vertical shaft up and down. Like what shown in the image.

So you can see that when the cam is reaching the peak point, the load on the axis is the greatest hence the master axis drive slow down and when the peak is over the speed will pick up again.

Then I cannot get consistent speed over 1 revolution of the cam. Especially at slow speed on the master axis.

The inverter we use for the master axis is an AB powerflex 40.
I have try to use the parameter A084 [Boost Select] but not success.

Regards

Click to expand...

A PowerFlex 40 is a sensorless vector, and that explains your speed regulation variation. A better (Rockwell) choice would be a PowerFlex 7(x). You will get better speed regulation. Also, if you might oversize the drive, and possibly add regenerative braking, that might give the drive better response.

- Improving speed regulation is a good choice. Like you said, a servo, or like jdbrandt said, a better drive.
- Sometimes a faster motor RPM will allow inertia to help through "slow spots" (if you can use a different gear ratio).
- If you are trying to use an encoder and virtual axis, what is the purpose of the virtual axis? Meaning, what is the output controlling? The only way I know to connect an encoder to a virtual axis is through PLC instructions. The performance of this is probably too low to use in a motion task.
- What is driving the vertical motion?
a) If this is a servo, then you should be using a servo for the rotary axis also. Don't waste time.
b) If this is not a servo, maybe you can use speed compensation to change the trigger timing (calculate trigger time based on speed).

It the Powerflex that drive the cam which in turn drive the vertical motion.

What I have in mind by mean of virtual axis is to
Apply MoveVelocity to the virtual axis and at the same time
Apply the same velocity set point to the drive.
I can foresee a few problems eg when the velocity
Set point changes the rate of change for both virtual
And master axis must be the same.

Then if the virtual and master have about the same velocity
Then using position additive offset to make the virtual to be in
Phase with the master axis.

Again this is only in the research phase.
But I hope to be able to do this without changing
The current hardware setup.

Regards

The master axis in turn is a master to a feeder axis
Then feed at certain angle of the master.
Then you know the problem when the feeder when
The master axis is in the slow region.

If the concept work then I will use the virtual axis as the master
To the feeder axis.
Forgive me I forgot to mention the feeder axis

Regards

The feeder is GearIn to the master

can you post a sketch the layout your motors,encoder

so is the issue that the "feed in" axis (geared to the encoder on the Master) when you are running slow is too early feeding in product?

If so this is easily fixed by phase shifting the slave when running slow (very common issue) (ie changing the master/slave MAG lock position based on speed)